It is necessary, for certain types of equipment, to determine the magnitude of errors introduced as a function of frequency by a circuit employed in that equipment. The circuit is also usually employed over a several amplitude ranges. For example, calibrators for deriving AC calibrating voltages that are applied to AC voltmeters to be calibrated include variable gain amplifiers responsive to a variable frequency AC source. The gain of the amplifier is varied as a function of the range of the AC output signal of the calibrator, while the frequency of the oscillator is varied as a function of the desired output frequency of the calibrator. In the calibrator specifically disclosed in the co-pending application, the amplitude and frequency ranges are respectively between 200 millivolts and 1,100 volts and 10 Hz and 1 MHz.
AC voltmeters incorporate similar variable gain amplifiers designed to be responsive to AC voltages, over wide amplitude and frequency ranges, e.g., between 200 millivolts and b 1,100 volts and frequencies between 10 Hz and 1 MHz. Amplifiers employed in AC meters must be capable of scaling the AC voltage applied to input terminals of the meter to a level which can be handled by a precise AC-to-DC detector connected to output terminals of the amplifier. The detector is usually capable of accurately handling voltages only over a relatively narrow range of AC output voltages of the amplifier, for example, 0 to 20 volts. Hence, the amplifier must be capable of providing gain and attenuation of the AC input signal over a wide range of input amplitudes and frequencies.
The variable gain amplifiers employed in calibrators and AC voltmeters employ, as the gain determining elements thereof, resistors which are variable in discrete steps, as a function of the range of the voltage to be handled by the amplifier. The resistors of different amplifiers have different values and are subject to change in values as a function of time, temperature and other environmental factors. The different values of the resistors change the gain of the amplifiers with which they are associated so the output voltages of the amplifiers for a particular amplitude differs from what it is supposed to be. Changing the amplitude of the output voltages of the variable gain amplifiers employed in calibrators and voltmeters thereby causes errors in the amplitude of the calibrating voltage applied by the calibrator to a meter being calibrated and the output reading of the meter.
Amplifiers employed in calibrators and meters for ranging, i.e., scaling, also have variable amplitude versus frequency responses. Each amplifier includes distributed reactances, usually in the form of shunt capacitors tending to make the amplifier output voltage decrease as a function of increasing frequency. To enable accurate AC outputs to be derived by calibrators and accurate readings to be obtained from AC voltmeters, it is necessary to determine the amplitude versus frequency response of the amplifiers and other circuits included in the calibrator and voltmeters.
Numerous switches and complex manual procedures, which are time consuming and subject to error, have been previously used to determine the calibrating errors for different ranges and frequencies of circuits employed in AC calibrators and AC voltmeters. The requirement for numerous switches has introduced errors in the calibration process due to the voltage drops associated with the switches.
An object of the invention is to provide a new and improved meter for providing accurate indications of AC voltage over a wide range of amplitudes and frequencies and calibration errors are compensated in a meter scaling circuit as a function of the meter range setting and the frequency of the AC voltage applied to the scaling circuit.
In accordance with an aspect of the invention, a digital meter for measuring the amplitude of AC voltages over several ranges includes a variable gain amplifier connected to be responsive to AC voltages applied to input terminals of the meter. The gain of the amplifier is controlled by resistor means included therein. The resistor means is variable in discrete steps in response to a range input signal for the meter. The gain of the amplifier is different for different frequencies of the AC voltages applied to the input terminals, typically because of parasitic reactances included in the amplifier. A digital signal having a value indicative of an AC output voltage of the amplifier is derived, in the preferred embodiment by an RMS-to-DC converter that drives an analog-to-digital converter. Frequency detector means responsive to the AC voltages applied to the input terminals derives a signal indicative of the frequency of the AC voltage applied to the input terminals. In response to the range input signal and the frequency indicating signal, a digital calibration signal for the amplitude of the amplifier output is derived in the preferred embodiment by a microcomputer. The microcomputer includes means for storing amplitude calibration signals for the output voltage of the amplifier as a function of range settings of the amplifier and the frequency of the AC voltage applied to the amplifier. The digital signal indicative of the AC output voltage of the amplifier is modified by the digital calibration signal to derive a corrected digital signal for the AC voltage applied to the input terminals. Digital indicator means responds to the corrected digital signal to provide a precise visual display of the AC voltage applied to the meter.
In accordance with another aspect of the invention, a digital meter for measuring the amplitude of AC voltages over several amplitude ranges comprises a variable gain amplifier connected to be responsive to AC voltages applied to input terminals of the meter. The gain of the amplifier, which is subject to being different for different frequencies of the AC voltages applied to the input terminals, is controlled in discrete steps in response to a range input signal. Digital signal processing means, including a micro-processor and a memory, responds to the range input signal, a signal indicative of the frequency of the AC voltage applied to the input terminals, and a signal indicative of the amplitude of the AC voltage at output terminals of the amplifier. The signal processing means responds to signals stored in the memory thereof to (1) supply a range dependent control signal to a gain controller for the amplifier, and (2) derive a digital signal indicative of the value of the AC voltage applied to the input terminals, as indicated by the AC voltage amplitude at the amplifier output terminals and corrected for the different gains of the amplifier as a function of frequency and range settings.
In accordance with a further aspect of the invention, apparatus for deriving a digital indication of the amplitude of an AC signal susceptible to multiple amplitude ranges and differing frequencies comprises a variable gain amplifier connected to be responsive to the AC signal. The amplifier has differing gain values as a function of the differing frequencies within the amplitude ranges and differing gain values as a function of the multiple ranges. The differing gain values affect the amplitude of the AC voltage at an output terminal of the amplifier. The amplitude of the AC voltage at the output terminal and the frequency of the AC signal amplified by the amplifier are detected. Digital signal processing means responsive to the detected amplitude and frequency derives the digital indication with compensation being provided for the functions of differing gain values.
In accordance with another aspect of the invention, apparatus for deriving a digital indication of the amplitude of an AC signal susceptible of differing frequencies comprises a variable gain amplifier connected to be responsive to the AC signal. The amplifier has differing gain values as a function of the differing frequencies; the differing gain values affect the amplitude of the AC voltages at an output terminal of the amplifier. The amplitude of the AC voltage at the output terminal and the frequency of the AC signal amplified by the amplifier are detected. Digital signal processing means responds to the detected amplitude and frequency to derive the digital indication with compensation being provided for the functions.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.